Juday—Lakes in Central America. < 229 
which takes place in this layer aids in maintaining a supply of 
oxygen. But the water which lies below the zone,of circulation 
is not able to replenish its losses of oxygen until the next period 
of overturning and circulation. 
Table II, (p. 244) shows that the bottom waters of lakes Am¬ 
atitlan and Ilopango contained a distinctly smaller amount ; of 
dissolved oxygen than their surface waters. The difference be¬ 
tween surface and bottom in the former lake was 2.5 cc. per 
liter of water and in the latter almost 2 cc. In both lakes the 
most marked decrease came in the 10-15 meter stratum. There 
was an appreciable difference between surface and bottom in 
lakes Atitlan and Coatepeque, but it was not nearly so marked 
as in the other two lakes. In the former it amounted to only 
about 0.5 cc. and in the latter about 0.6 cc. 
As the season progresses the difference between surface and 
bottom undoubtedly becomes more marked in all of the lakes. 
In fact, it seems very probable that the dissolved oxygen is 
entirely exhausted in some of the lower water of both Amatitlan 
and Ilopango lakes before the autumnal overturning takes place. 
In lake Atitlan the volume of the lower water is relatively very 
great so that it holds a correspondingly large amount of oxy¬ 
gen in solution; the amount is so large, in fact, that the demands 
for oxygen in the lower strata may not be great enough to en¬ 
tirely exhaust the supply in any of this water. 
Downes 1 found that the water in some of the reservoirs of the 
Panama Canal Zone was permanently stratified and that, below 
a depth of 3 m., there was practically no dissolved oxygen 
throughout the year. 
The percentages of oxygen saturation shown in the last col¬ 
umn of table II (p. 244) are based upon the quantity of oxygen 
required for the saturation of perfectly fresh or distilled water 
as shown by Fox’s 2 determinations. It will be noted that these 
percentages are low, even at the surface where the water is= 
freely exposed to the air. The highest percentage of saturation 
was found in lake Amatitlan and the lowest in lake Atitlan. 
The quantity of oxygen absorbed by the waters of these lakes' 
is affected by two factors, namely, the salinity of the waters and 
the elevation of the lakes above sea level. The presence of 
sodium chloride reduces the capacity for oxygen. Chemical 
1 Proc. Med. Asso. of Isthmus of Panama, vol. Ill, p. 133-150, 7 pis. 1911. 
2 Publications de circonstance, No 1 . 41, Part I, 23 p.,-1 pi. 1907. 
